It most likely means “what causes his mind to wander?” In plain English, that refers to his attention drifting away from the task or conversation to unrelated thoughts, which is commonly called mind wandering.

Simple meaning

  • Mind wander = to daydream, zone out, or get lost in thoughts.
  • It can happen because someone is bored, distracted, stressed, or mentally overloaded.
  • It can also be a normal brain state when attention is not fully locked onto one task.

Example

  • “During the lecture, his mind wandered” means he stopped focusing on the lecture and started thinking about something else.

Word note

  • The phrase you wrote, “cause his mind to wonder,” is probably a mix-up.
  • Wonder means to think curiously or be amazed, while wander means to drift off-task.

Bottom line

If you meant the common phrase, the answer is: something is causing his attention to drift away from what he should be focusing on.